Synchronization method in a cellular telecommunication network hand-over method corresponding terminal and base station

ABSTRACT

The invention concerns synchronization of a terminal in a cellular communication network comprising a plurality of cells, whereof a first and a second cell associated respectively with a first and a second base station, the first and second cells at least partly overlapping in a common geographical zone and being synchronized, the synchronization comprising: a first synchronization step on a synchronizing signal transmitted by the first base station and received by the terminal; a second synchronization step on a predetermined signal dedicated to multipath processing, transmitted by the second base station and received by the terminal.

This invention is related to cellular radiotelephony. More precisely,the invention relates to synchronisation of a terminal on a mobile radiocommunication network cell.

In a cellular network, for example a UMTS (“Universal MobileTelecommunication System) type or a 3GPP (“3^(rd) Generation PartnershipProject”) type cellular network, equipment must be synchronised on acell before it can transmit and/or receive data quickly. Therefore, inorder to optimise information transmissions from or to terminals, it isnecessary to enable fast synchronisation of terminals on a cell in thenetwork.

Synchronisation techniques are known based on the use of a dedicatedchannel, for example of the SCH (“Synchronisation CHannel”) type. Thissynchronisation is usually relatively effective but it has thedisadvantage that it is slow under some circumstances, particularly whenthe equipment moves from one cell to another and changes frequency.

Furthermore, in order to take account of the mobility of equipment in acellular network without interrupting communications in progress, thenetwork must manage cell transfers or “hand-overs” that enable equipmentto change from cell to cell very quickly. Note that a “hand-over” isused particularly when it is preferable for terminal in communicationwith a fixed station (Base Station BS responsible for the management ofa cell) to change the base station with which it is associated, tocommunicate. A “hand-over” can be used for different reasons: badquality of radio links, saturation of a base station due to too manycommunications, mobility of the equipment that moves away from a firststation and towards another station, better adaptation of a base stationfor the service requested by the terminal, etc.

This “hand-over” must be made quickly, for reasons of efficiency,quality and fluidity of the service offered to the user.

Conventionally, a relatively slow inter-frequency “hand-over” procedureis used (in other words when the frequencies of cells to which equipmentis attached are different before and after the “hand-over”) whichgenerates interference.

In order to make an inter-frequency “hand-over” in a third generationradiotelephony system using a channel access technique based on a CDMA(Code Division Multiple Access), for example in a UMTS or 3GPP system, amobile equipment has to use vacant periods in “compressed mode” tosynchronise itself onto different cells in order to make powermeasurements on these cells. For synchronisation, the mobile equipmentmust listen to the primary SCH channel, then the secondary SCH channel,then the CPICH channel on which it makes the power measurement.

This technique according to prior art has the disadvantage that it has avery long synchronisation phase since vacant periods output fromcompressed mode are short duration.

Therefore the UMTS and 3GPP networks are defined such that beforeinitialising a communication, the mobile equipment is already on anoptimum cell which avoids the need for a “hand-over” at the beginning ofa communication.

The various aspects of the invention are intended particularly toovercome these disadvantages in prior art.

More precisely, a first purpose of the invention is to enable fastsynchronisation when equipment moves from one cell to another.

Another purpose of the invention is to enable a fast “hand-over” ofequipment particularly in the case of an inter-frequency “hand-over”procedure.

Another purpose of the invention is to enable the use of equipmentintended for third generation mobile communication networks, requiringno or few modifications to current standards in force and particularlythe UMTS FDD (Frequency Division Duplex) standard (in particular, series25 of this standard) defined and distributed by the 3GPP (“3^(rd)Generation Partnership Project”) committee.

In order to achieve this, the invention proposes a method forsynchronisation of a terminal in a cellular communication networkcomprising several cells, including a first cell and a second cellassociated with a first and a second base station respectively, thefirst and second cells at least partly intersecting over ageographically common area and being synchronised, and remarkable inthat the method includes the following steps, when the terminal ispresent in the geographically common area:

-   -   a first synchronisation step onto a synchronisation signal (SCH)        transmitted by the first base station and received by the        terminal;    -   a second synchronisation step on a predetermined signal (CPICH)        dedicated to the processing of multiple paths, transmitted by        the second base station and received by the terminal.

In this case, the first and second cells are perfectly synchronised witheach other or are pseudo-synchronised, in other words are synchronisedwith some synchronisation tolerance.

The first synchronisation of the terminal is perfect or quasi-perfectrelative to the first base station, while it may be coarse on the secondbase station.

On the other hand, the second terminal synchronisation is perfect orquasi-perfect relative to the second base station.

According to one particular characteristic, the synchronisation methodis remarkable in that the equipment implements a step to determine atleast one path corresponding to the predetermined signal transmitted bythe second base station, the determination step supplying the secondsynchronisation step and being implemented by means of analysing themultiple paths along which the predetermined signal passes, the path orone of the paths corresponding to the predetermined signal, called thefirst path, being considered as the synchronisation base.

Thus, synchronisation is relatively easily implemented on the secondbase station according to the invention because it uses means that havealready been implemented to analyse multiple paths affecting thepredetermined signal (for example the CPICH channel for the UMTSstandard).

Furthermore, the invention saves pass-band and simplifies implementationon the second base station since there is no need for a specificsynchronisation channel (for example the SCH channel for the UMTSstandard).

According to one particular characteristic, the synchronisation methodis remarkable in that the first synchronisation step toleratessynchronisation errors of the order of 5 to 50 μs relative to the secondbase station.

According to one particular characteristic, the synchronisation methodis remarkable in that the first synchronisation step toleratessynchronisation errors of the order of 5 to 30 μs relative to the secondbase station.

According to one particular characteristic, the synchronisation methodis remarkable in that the second synchronisation step toleratessynchronisation errors of less than 5 μs relative to the second basestation.

Thus, the invention enables a relatively coarse first synchronisation ofthe terminal with respect to the second base station (up of 30 μS oreven 50 μs) and a second fine or even perfect synchronisation of theterminal with respect to the same base station.

According to one particular characteristic, the synchronisation methodis remarkable in that the first base station transmits synchronisationinformation to the second base station enabling synchronisation of thesecond base station on the first base station.

According to one particular characteristic, the synchronisation methodis remarkable in that the synchronisation information is transmitted ona wire link.

According to one particular characteristic, the synchronisation methodis remarkable in that the synchronisation information is transmittedthrough the synchronisation signal transmitted by the first basestation.

According to one particular characteristic, the synchronisation methodis remarkable in that a third piece of equipment transmitssynchronisation information to the first base station and to the secondbase station enabling synchronisation of the second base station on thefirst base station.

Thus, it is relatively simple to implement synchronisation between thefirst and second base stations.

In particular, it may be implemented:

-   -   through an existing synchronisation channel (for example SCH for        the UMTS) when the second base station picks up this signal        transmitted by the first base station; or    -   by a dedicated wire link or using the network infrastructure        which overcomes any problems with transmission/reception of a        radio signal; or    -   using a third party equipment that outputs a synchronisation        signal to the first station and to the base stations, which        enables a fine control of synchronisation by a third equipment        that may be dedicated to this purpose, or for example that may        be connected to or form part of control equipment in the first        and second base stations.

According to the invention, the synchronisation difference may also becontrolled such that the first synchronisation of terminal on the firstbase station is as fine as possible with respect to the second basestation. In particular, the propagation delay of the synchronisationsignal transmitted by the first base station to the second base stationcan be taken into account. A network equipment (first base station orcontrol equipment) could also inform the terminal about thesynchronisation delay between the two base stations so that the terminalcan take it into account.

The invention also relates to a method of hand-over of a terminal in acellular communication network comprising several cells, including afirst cell and a second cell associated with a first base station and asecond base station respectively, the first and second cells at leastpartly intersecting over a geographically common area and beingsynchronised, and in particular the terminal being in communication modewhen a communication has been set up between the terminal and a remoteterminal, and in standby mode when the terminal is not in communicationmode but is present and is available for a communication in one of thenetwork cells, and is remarkable in that the terminal is present in thegeographically common area, the method includes:

-   -   a first synchronisation step onto a synchronisation signal (SCH)        transmitted by the first base station and received by the        terminal;    -   a second synchronisation step on a predetermined signal (CPICH)        dedicated to the processing of multiple paths, transmitted by        the second base station and received by the terminal;    -   when the terminal in communication mode with the first base        station is transferred from the first cell to the second cell.

Thus, the invention enables terminal to be transferred between two cells(or a fast “hand-over”) even if the first and second base stations donot use the same frequencies.

According to one particular characteristic, the cell-to-cell transfermethod is remarkable in that it uses a synchronisation method like thatdescribed above.

According to one particular characteristic, the cell-to-cell transfermethod is remarkable in that the first cell surrounds the second celland in that the first base station manages standby mode for terminalspresent in the first cell, the second base station being able to managecommunication mode.

According to one particular characteristic, the hand-over method isremarkable in that the first base station manages opening of acommunication for terminals present in the second cell, and the networkthen transfers management of the communication to the second basestation.

The invention also relates to a terminal that will cooperate with atleast a first base station associated with a first cell in the cellularcommunication network, in particular with the terminal possibly incommunication mode, when a communication is set up between the terminaland a remote terminal, and in standby mode when the terminal is not incommunication mode but is present and available for a communication inone of the network cells, and is remarkable in that it comprises:

-   -   means of making a first synchronisation on a synchronisation        signal (SCH) transmitted by the first base station and received        by the terminal;    -   means of making a second synchronisation on a predetermined        signal (CPICH) dedicated to processing of multiple paths,        transmitted by a second base station and received by the        terminal;

a second cell in the network being associated with the second basestation; and the first and second cells at least partly intersecting andbeing synchronised.

According to one particular characteristic, the terminal is remarkablein that the synchronisation means include means of analysing multiplepaths followed by the predetermined signal, capable of determining atleast one path corresponding to the predetermined signal transmitted bythe second base station, the path or one of the paths corresponding tothe predetermined signal, called the first path, being considered assynchronisation base.

According to one particular characteristic, the terminal is remarkablein that the first synchronisation tolerates synchronisation errors ofthe order of 5 to 50 μs.

According to one particular characteristic, the terminal is remarkablein that the first synchronisation tolerates synchronisation errors ofthe order of 5 to 30 μs.

According to one particular characteristic, the equipment is remarkablein that the second synchronisation tolerates synchronisation errors ofless than 5 μs.

The invention also relates to a base station, called the second basestation, that will be associated with a second cell in the cellulartelecommunication network, the network comprising several cellsincluding a first cell associated with a first base station and at leastone terminal, the first and second cells intersecting at least partly ona geographically common area, remarkable in that the second base stationcomprises means of synchronisation on the first base station such thatat least one of the terminals can implement:

-   -   a first synchronisation step onto a synchronisation signal (SCH)        transmitted by the first base station and received by the        terminal; and    -   a second synchronisation step on a predetermined signal (CPICH)        dedicated to the processing of multiple paths, transmitted by        the second base station and received by the terminal.

According to one particular characteristic, the base station isremarkable in that it comprises means of management of a terminalcommunication mode and that it does not include means of management of aterminal standby mode, in particular the equipment possibly being incommunication mode when a communication is set up between the terminaland a remote terminal, and in standby mode when the terminal is not incommunication mode but is present and available for a communication inone of the network cells.

Moreover, the invention relates to a cellular telecommunication networkcomprising several cells, including a first cell associated with a firstbase station and at least one second cell, the first cell and each ofthe second cells intersecting at least partly on a geographically commonarea and being synchronised, remarkable in that the second cell(s) is(are) each associated with at least one second base station like thatdescribed above.

The advantages of the hand-over method, the terminal, the second basestation in the network are the same as the advantages of the terminalsynchronisation method, and will not be described in more detail here.

Other characteristics and advantages of the invention will becomeclearer after reading the following description of a preferredembodiment, given as a simple illustrative example that is in no waylimitative, with reference to the attached drawings among which:

FIG. 1 shows a block diagram of the network according to a particularembodiment of the invention;

FIG. 2 illustrates the network in FIG. 1 after a communication has beenset up between a terminal and a base station associated with amicro-cell;

FIG. 3 describes a “micro-cell” base station in the network illustratedin FIGS. 1 and 2;

FIG. 4 illustrates a communication protocol between different elementsof the network enabling the change over from a situation illustrated inFIG. 1 to a situation illustrated in FIG. 2;

FIG. 5 shows a block diagram of the network according to a variantembodiment of the invention; and

FIG. 6 illustrates a communication protocol between different elementsof the network in FIG. 5 during a fast “hand-over” procedure.

The problem of changing cells is related to the necessary temporalre-synchronisation of a terminal in communication on the new cell.

The general principle of the invention consists of simplifying thisoperation and is therefore based particularly on:

-   -   synchronisation or pseudo-synchronisation of adjacent cells or        partially or entirely superposed cells (particularly the case of        a large cell surrounding a small cell); and    -   the fact that the distance between a terminal and a base station        in the cell to which the “hand-over” is being made is        sufficiently small to consider that the terminal does not need        to listen to synchronisation channels (SCH) of this base        station.

Note that synchronisation between the cells according to the approachused in the invention does not need very high precision andpseudo-synchronisation may be sufficient. Pseudo-synchronisation in thiscontext means synchronisation with a precision of less than 50 μS andpreferably less than or equal to 30 μs.

In particular, this pseudo-synchronisation may be obtained by:

-   -   using a wire link between two stations each associated with one        of the two cells (this link possibly being dedicated to        synchronisation or a link used to transport data); or    -   a first base station listening to and processing the        synchronisation signal transmitted by a second base station on a        synchronisation channel.

Note that user terminals consist particularly of mobile or fixedwireless equipment (for example mobile telephones or any other equipment(for example portable computers) containing a wireless communicationsystem).

There is no need for two cells to be re-synchronised with respect toeach other frequently when the cells can be pseudo-synchronised, by afirst base station attached to a first cell listening to and processingthe synchronization signal transmitted by a second base station attachedto a second cell on a synchronisation channel (SCH) and taking accountof the very small drifts in the frequency references of the basestations.

Furthermore, since the distance between the terminal and a base stationto which the “hand-over” procedure is used is relatively small,consequently the time uncertainty for synchronisation is short andtherefore the synchronisation phase of the destination cell during the“hand-over” takes place only by listening to the CPICH (Common PIlotCHannel) channel and not the SCH channel, these two channels beingassociated with the base station of the destination cell.

The CPICH channel is used according to the UMTS standard to determinethe phase and to search for the different echoes of a synchronised cell.

In the context of the invention, the CPICH channel is also used as asynchronisation channel.

Note that the electronic part of the terminal used to search for thedifferent echoes of a synchronised cell may be used to synchronise on acell if asynchronism is not too great.

UMTS terminals are designed to search for the different echoes of a cellat about plus or minus 26 μs around the known echo, that istheoretically the direct path if there is one.

Therefore, the use of the CPICH channel provides a means ofsynchronising a cell for which synchronisation is known to within plusor minus 26 microseconds.

In the context of the invention, these 26 microseconds must correspondto the synchronisation error between the two cells (the two cells may bepseudo-synchronised as mentioned above), and to the uncertainty relatedto the unknown propagation time in the destination cell.

Therefore, it is important that the destination cell should be small,for example the propagation time in a cell with a radius of 3 km isbetween 0 μs (if the terminal is very close to the base station) andabout 10 μs if the terminal is at the boundary of the cell.

It is important to note that the invention does not require that allcells in UMTS networks should be adapted. Some cells in the same networkmay operate using the mechanism according to the invention, while othercells are not pseudo-synchronised.

We will now describe a block diagram of a mobile radiotelephony networkusing the invention, with reference to FIG. 1.

For example, the network may be compatible with the UMTS (UniversalMobile Telecommunication System) standard defined by the 3GPP committee.

The network includes a large cell 100 (macro-cell) managed by a basestation 101 (BS).

This cell 100 surrounds two smaller cells 110 and 120 (“micro-cell” or“pico-cell”).

Each of the cells 110 and 120 comprises a base station 111 and 121respectively, that can manage communications inside the correspondingcell.

Note for illustration purposes that several items of equipment (UE) arepresent inside cell 100. Some of these items of equipment are alsopresent. in one of the small cells 110 and 120.

Thus, the terminal 112 is inside the cell 110 and can therefore receiveor transmit signals from or to the base stations 101 and 111.

Similarly, terminals 122 and 123 are inside cell 120 and can thereforereceive or transmit signals from or to the base stations 101 and 121.

However, terminals 102 and 103 present in the cell 100 but not presentin one of cells 110 and 120 can either transmit signals from or to thebase station 101 but not from or to base stations 111 or 121.

In FIG. 1, the connections between the different elements of cell 100are represented, at a given instant:

-   -   in thin dashed lines for connections between base stations;    -   in thick dashed lines for connections between the base station        101 and the terminals in standby state (terminal 112, 122, 123        and 102 according to the example in FIG. 1); and    -   in solid lines for communication links (link between terminal        103 and the base station 101).

Note that some terminals are in standby mode, in other words in a modein which the terminal is not in communication mode but is present andavailable for a communication in one of cells 100, 110 or 120. Inparticular, these terminals are listening to signals transmitted by thebase station 101 belonging to macro-cell 100. These signals aretransmitted on:

-   -   common transport channels corresponding to services offered to        high layers of the communication protocol, particularly on BCH        (“Broadcast CHannel”) channels and PCH (“Paging CHannel”)        channels; and    -   common transport channels corresponding to the physical layer of        the communication protocol, particularly on CPICH (“Common PIlot        CHannel”) channels.

Note also that in standby mode, terminals are not listening to thededicated channels.

On the other hand, the terminal 103 is not in standby mode because it isin communication with the base station 101 on a Dedicated CHannel (DCH)which is an up and down channel at the same time.

The channels used by 3GPP networks are well known to those skilled inthe art for mobile networks and in particular are specified in the“3^(rd) Generation Partnership Project, Technical Specification GroupRadio Access Network; Physical Channels and mapping of transportchannels onto physical channels (FDD) release 1999” standard reference3GPP TS25.211 and published by the 3GPP publications office. Therefore,these channels are not described here in more detail.

FIG. 2 shows the network in FIG. 1 when some time has elapsed andnotably after a communication has been set up between the terminal 123and the base station 121 inside the micro-cell 120.

Note that according to FIG. 2, the terminal 123 is directly connected tothe base station 121 through an up or down dedicated channel DCHenabling transport of the channel and/or exchanged data.

FIG. 3 diagrammatically illustrates the base station 121 as illustratedwith respect to FIGS. 1 and 2.

The base station 121 comprises the following, connected to each other byan address and data bus 307:

-   -   a processor 304;    -   a RAM 306;    -   a non-volatile memory 305;    -   a wire network interface 300 making a connection to a fixed        infrastructure of the mobile network or to other networks;    -   a radio reception interface 301 for receiving signals        transmitted by terminals in communication with the base station        121 on dedicated up channels and signals transmitted by the base        station 101, notably on the Synchronisation CHannel SCH (note        that the existing UMTS standards do not require that the SCH        channel is listened to only by user equipment and not by a base        station);    -   a transmission radio interface 302 for transmitting signals on        dedicated down channels and on common transport channels        corresponding to the physical layer (and not to upper-layers of        the communication protocol) (particularly CPICH channel); and    -   a man/machine interface 303 enabling a dialog with the machine        for control and maintenance.

The RAM 306 stores data, variables 309 and intermediate processingresults.

The non-volatile memory 305 keeps the following in registers which, forconvenience, have been given the same names as the data stored in them,and among other:

-   -   the operating program of the processor 304 in a “prog” register        310 and    -   configuration parameters 311 for the base station 121.

Note that the base station 121 is implemented more easily than the basestation 101 and in particular includes a simpler operating program thanthe operating program of the base station 101 since it does not includecommon channel functions that the base station 121 does not need tomanage.

Note that a terminal (not shown) includes the following, connected toeach other by an address and data bus:

-   -   a processor;    -   a RAM;    -   a non-volatile memory;    -   a radio reception interface for synchronising in standby mode        onto an SCH type signal transmitted by the base station 101, and        then in communication mode onto a CPICH type signal transmitted        by the base station 121 and in general receiving signals        transmitted by the base stations 101 and 121 on dedicated down        channels;    -   a transmission radio interface for transmitting signals on        dedicated up channels and on common up transport channels; and    -   a man/machine interface enabling a dialog with the machine for        control and maintenance.

FIG. 4 illustrates a communication protocol between base stations 101and 121 and the terminal 123 when changing from the situationillustrated with reference to FIG. 1, in which the equipment 123 is instandby mode, to a situation illustrated with reference to FIG. 2, inwhich the equipment 123 is in communication with the base station 121.

The base station 101 transmits a signal 400 on the SCH down channel tobase stations and terminals present in the macro-cell 100 andparticularly from the base station 121 and the terminal 123. Thus, thebase station 121 and the terminal 123 (which according to FIG. 1 is instandby mode) are synchronised on the SCH channel of the base station101.

Note that the base station 101 transmits this SCH signal regularly andthat as soon as the pseudo-synchronisation of the base station 121degrades beyond a certain predetermined threshold, the base station 121resynchronises itself on the base station 101.

Note also that the base stations 101 and 121 are fixed and thereforethat the signal propagation time between these two stations is known.Thus, knowledge of this propagation time can be used to improve thesynchronisation of the terminal on the base station 121, by using:

-   -   a synchronisation delay of the base station 121 with respect to        this SCH signal transmitted by the base station 101, for example        this delay being equal to the propagation time of the SCH signal        between base stations 101 and 121; and/or    -   a “hand-over” signal (signal 405 described in detail later)        transmitted to the terminal 123 and transporting information        indicating the synchronisation position.

The base station 101 also transmits a signal 401 on the BCH channel.This down signal indicates which PCH channel the terminal 123 shouldlisten to. Thus, after reception of this signal, the terminal 123listens to the PCH channel indicated by the signal 401.

The base station 101 then transmits a signal to terminal 123 on the PCHchannel indicated by the signal 401, this signal being used to detect anincoming call.

Then, assuming that the terminal 123 wants to initialise acommunication, it transmits a signal 403 on the RACH channel (“RandomAccess CHannel” which is a common channel corresponding to a high layerchannel access service), this signal 403 notifying the base station 101that the terminal 103 is requesting that a communication should be setup.

The base station 101 then transmits a communication channel allocationsignal 404 on the FACH channel (“Fast Access CHannel” which is also acommon channel corresponding to a high layer service).

Communication is then set up between the terminal 123 and the basestation 101. One or several signals 405 containing data corresponding toan equipment application and then control data dedicated to the handoverare thus exchanged on the bi-directional channel DPCH.

Note that the hand-over for carrying a communication from terminal 123to the base station 121 is made following a network decision(particularly by the RNC (Radio Network Controller) connected to basestations 101 and 121) as a function of many criteria, particularly thethroughput, the communication quality and specific features of the basestation 121 (particularly the fact that it is well adapted to managinghigh throughput communications).

The network situation then becomes like that illustrated with respect toFIG. 2.

The terminal 123 then listens to the pilot channel 406 CPICH which,according to the invention, is used to refine the synchronisation ofterminal 123. If cell 120 is small and the base station 121 ispseudo-synchronised on station 101 (in other words if synchronisationbetween cells 120 and 100 is coarse and imperfect, the synchronisationerror being less than about 20 or 30 μs, then the synchronisation errorin the synchronised networks, known in themselves, is less than 5 μs),the resulting synchronisation error between the terminal 123 and thebase station 121 may be compensated by using the signal 406. Theterminal 123 comprises means of making use of the multiple pathsaffecting a signal transmitted by a base station (this multiple pathsphenomenon is well known to those skilled in the art and is the resultin particular of reflections of a signal on obstacles and transmitted inseveral directions, the different received signals originating from thesame transmitted signal but that have followed different paths, ingeneral have different amplitudes and are out of phase). Note inparticular that a “rake” type receiver can be used to determine thedifferent delays affecting a multi-path signal. Thus, if the delay isnot too large (in other words is less than 20 μs in the context of the3GPP standard), the equipment 123 is capable of synchronising itself onthe base station using the CPICH channel.

Thus, assuming that a first path is located at a precise instant thatdepends on the synchronisation with the base station 101, the receiverof terminal 123 fixing itself on this hypothetical path searches for atleast one path corresponding to a signal transmitted on the CPICHchannel of the base station with means also used for the determinationof multiple paths in a signal transmitted on a CPICH channel. This ispossible because synchronisation differences between the terminal 123and each of the base stations 101 and 121 are small. The path or one ofthe determined paths is then used as the basis for synchronisation(synchronisation base) of the terminal 123 onto the base station 121.

Note that in the context of the 3GPP standard, the CPICH can be used toprocess multi-paths with a delay of 20 μs, which provides a means ofcompensating for an error when the radius of the small cell is less thanor equal to about 6 km (namely the delay equal to approximately 20 μs inthis case multiplied by the speed of light).

Note also that when it is synchronised on the base station 121, theterminal 123 maintains slaving on this synchronisation through the CPICHchannel managed by the base station 121.

The terminal 123 and the base station 121 then exchange data on thededicated channels DPCH through several signals 407 to 409, of whichonly a small part has been shown.

At the end of the communication, the terminal 123 and/or the basestation 121 indicate that the communication has terminated, through thesignal 409.

According to one variant that has not been shown, the network imposesthat the terminal should make a “hand-over” to the base station 101before the end of communication. Note that this “hand-over” can be madequickly with synchronisation on the CPICH signal transmitted by the basestation 101 since the terminal is synchronised on the base station 121which is itself pseudo-synchronised on the base station 101.

Therefore, the equipment 123 goes back into standby mode and thesituation then returns to the situation illustrated with reference toFIG. 1.

The base station 101 then transmits signals 410, 411 and 412 on the SCH,BCH and PCH channels respectively, these signals being similar tosignals 400, 401 and 402 respectively described above.

We will now describe a block diagram of the mobile radiotelephonynetwork in relation to FIG. 5, using one variant embodiment of theinvention.

For example, the network is compatible with the UMTS (“Universal MobileTelecommunication System”) standard defined by the 3GPP (“3^(rd)Generation Partnership Project”) committee.

The network includes two cells 500 and 510 that cover a geographicallycommon area 504, the cell 510 being sufficiently small to consider thatthe terminal 503 does not need to listen to synchronisation channels SCHof the station 510 to synchronise on this station when it issynchronised on station 501.

Each of the cells 500 and 510 includes a base station 501 and 511respectively, capable of managing communications inside thecorresponding cell.

Note, for illustration, that several items of equipment (UE) are presentin at least one of the cells 500 and 510, a terminal 503 being presentin the common area 504.

Thus, the terminal 503 can receive or transmit signals from or to thebase stations 501 and 511.

In FIG. 5, the connections between the different elements of the cell500 have been shown, at a given instant:

-   -   radio links shown in thin dashed lines for connections between        base stations 501 and 511 and terminals 502, 503 or 512; and    -   wire link 520 shown in solid lines between base stations 501 and        502.

For example, the wire link 520 is a dedicated line connecting the twobase stations 501 and 511 or part of a fixed network capable oftransporting synchronisation signals and possibly data.

Thus, note that the terminal 503 is in standby mode, in other words, itis in mode in which it is not in communication mode but it is presentand available for communication in one of cells 500 or 501. Therefore,the terminal 503 is listening particularly to signals transmitted by thebase station 501 belonging to cell 500. These signals are transmittedon:

-   -   common transport channels corresponding to services offered to        high layers of the communication protocol, particularly on BCH        (“Broadcast CHannel”) channels and PCH (“Paging CHannel”)        channels; and    -   common channels corresponding to the physical layer of the        communication protocol, particularly on the SCH channel and the        CPICH (“Common PIlot CHannel”) channel.

Note also that in standby mode, the terminals are not listening to thededicated channels.

Those skilled in the art for mobile networks will be familiar with thechannels used by 3GPP networks that in particular are specified in the“3^(rd) Generation Partnership Project; Technical Specification GroupRadio Access Network; Physical Channels and mapping of transportchannels onto physical channels (FDD) release 1999” standard reference3GPP TS25.211 and published by the 3GPP publications office. Thereforethese channels are not described in more detail here.

Furthermore, note that the base stations 501 and 511 are very similar tothe base station 121 illustrated with reference to FIG. 3, a wirenetwork interface enabling a link 520 to the other base station (511 and501 respectively) and to a fixed infrastructure of the mobile network orother networks.

Note also that the terminals used in this variant of the invention aresimilar to the terminals used in the first embodiment of the inventiondescribed above.

Therefore, base station or equipment type devices will not be describedin more detail here.

FIG. 6 illustrates a communication protocol between base stations 501and 511 and terminal 503 during a “hand-over” of terminal 503 from cell500 to cell 501.

According to the embodiment of the invention described in FIG. 5, thebase station 511 does not synchronise on the SCH channel of the basestation 501. The radio reception interface of the base station 521provides a means of receiving signals transmitted by terminals incommunication with the base station 511 on dedicated up channels anddoes not receive signals transmitted by the base station 501 on a radiomedium, particularly on the radio Synchronisation CHannel SCH when thebase station 501 is not in the common area 504.

On the other hand, the base station 511 receives a synchronisationsignal 615 transmitted by the base station 501 on the wire link 520.

The synchronisation signal 615 is used according to techniques known tothose skilled in the art (for example pulse according to a given rate ora particular bit sequence on which the base station 511 fixes its ownsynchronisation). Therefore, this synchronisation signal will not bedescribed in more detail here.

Note however that, according to the invention, this synchronisationsignal may take account of the propagation times of this signal tooptimise synchronisation of base stations with each other.

Note also that the fact that the terminal is located in a geographicallylimited area 504 may be used to improve synchronisation of the terminalon the base station 511. In particular, it is possible to take accountof the difference firstly in the propagation time between the terminal503 and the base station 501, and secondly the difference in thepropagation time between the terminal 503 and the base station 511, byusing compensation means and particularly:

-   -   a synchronisation signal controlling the base station 511 that        takes account of this difference; and/or    -   a “hand-over” signal (signal 610 illustrated later) transmitted        to equipment 123 and transporting information indicating the        synchronisation position.

Note also that the synchronisation signal 615 is regularly transmittedby the base station 501 and that as soon as the pseudo-synchronisationof the base station 511 degrades below a predetermined threshold, thebase station 511 re-synchronises itself on the base station 501 from thesignal 615.

The base station 501 also transmits a signal 600 on the down channel SCHto terminals present in the cell 500 and particularly terminal 503.Thus, terminal 503 (which is supposed to be in standby mode) issynchronised on the SCH channel of the base station 501.

The base station 501 also transmits a signal 601 on the BCH channel.This down signal informs the terminal 503 which PCH channel it shouldlisten to. Thus, after receiving this signal, the equipment 503 listensto the PCH channel indicated by the signal 601.

The base station 501 then transmits a signal to terminal 503 on the PCHchannel indicated by the signal 601, this signal being used to detect anincoming call.

Then, assuming that the terminal 503 wants to initialise acommunication, it transmits a signal 603 on the RACH channel (“RandomAccess CHannel” that is a common channel corresponding to a high layerchannel access service), this signal 603 notifying the base station 501that the terminal 503 is requesting that a communication should be setup.

The base station. 501 then transmits a communication channel allocationsignal 604 on the FACH channel (Fast Access CHannel, that is also acommon channel corresponding to a high layer service).

The communication is then set up between the terminal 503 and the basestation 501.

Thus, one or several signals 605 containing data corresponding to aterminal application are exchanged on the bi-directional channel DPCH.

At the same time, the terminal 503 listens to the CPICH channel of cell510 (signal transmitted by the base station 511) to make measurements ofthe power received on signals transmitted by the base station 511.

The terminal then transmits the result of this measurement to the basestation 501 by a DPCH type signal 607.

Terminal 503 then listens again to the CPICH channel of cell 510 and thepower measurement is repeated, during a step 608.

The terminal 503 then transmits the result of this measurement to thebase station 501 in a DPCH type signal 609.

The network (particularly the RNC (Radio Network Controller) connectedto base stations 501 and 511) then makes the decision to make a“hand-over” based on measurements communicated to the base station 501.Therefore the station 501 transmits a signal 610 transported on a DPCHchannel and notifying. the terminal 503 that it should listen to a DPCHchannel in cell 510.

Since the terminal 503 is synchronised on the SCH channel transmitted bythe station 501, the base station 511 being pseudo-synchronised onstation 501 and the cell 510 being small, the terminal is alsopseudo-synchronised on the station 511 with a synchronisation error notexceeding the sum of:

-   -   the difference between its own synchronisation on station 501        and the pseudo-synchronisation of station 511 on station 501;        and    -   the delay due to propagation of signals from station 511 to        terminal 503.

Note that this difference can be reduced by the use of compensationmeans illustrated above.

The terminal 503 then listens to the pilot channel 611 CPICH that,according to the invention, refines the synchronisation of terminal 503.If the cell 510 is small and if the base station 511 ispseudo-synchronised on station 501 (in other words if synchronisationbetween cells 500 and 510 is coarse and not perfect, the synchronisationerror being less than about 20 or 30 μs, then the error onsynchronisation in synchronised networks, known in themselves, is lessthan 1 or 2 μs), the resulting synchronisation error between theterminal 53 and the base station 511 can be compensated by using thesignal 611.

Note that this operation to synchronise terminal 503 on station 511takes place very quickly even if a frequency change is necessary during“hand-over”.

Note also that when the terminal 503 is synchronised on the base station511, it maintains slaving on this synchronisation through the CPICHchannel managed by the base station 511.

The terminal 503 includes means of taking advantage of the multiplepaths affecting a signal transmitted by a base station. In particular,note that a “rake” type receiver is capable of determining differentdelays affecting a multi-path signal. Thus, if the delay is not toogreat (in other words less than 20 μs in the context of the 3GPPstandard), the terminal 503 is capable of synchronising itself on theCPICH channel.

Note that in the context of the 3GPP standard, the CPICH is capable ofprocessing multi-paths with a delay of 20 μs, which provides a means ofcompensating for an error when the radius of the small cell is less thanor equal to about 6 km (namely in this case the delay equal to the orderof 20 μs multiplied by the speed of light).

The terminal 503 and the base station 511 then exchange data ondedicated channels DPCH by means of several signals 611 and 612, a smallpart of which has been shown.

At the end of the communication, the terminal 503 and/or the basestation 511 outputs signal 612 to indicate that the communication isterminated.

Therefore, the terminal 503 returns to standby mode.

The base station 511 then transmits signals 614 on the SCH, BCH and PCHchannels, these signals being similar to the signals 600, 601 and 602respectively described above.

Note that terminal 503 is still managed by cell 510.

According to one variant that is not shown, when the terminal is instandby mode and is in the common area 504, it listens to commonchannels specific to cell 500.

Obviously, the invention is not limited to the example embodimentsmentioned above.

In particular, those skilled in the art could make any variant to theway in which two cells are pseudo-synchronised. Thus, it could beconsidered that the base station of one cell receives a synchronisationsignal transmitted by a base station in another cell, for example usinga wire link or a radio link (particularly an SCH channel).

Nor is the invention limited to the case in which two base stations arepseudo-synchronised by means of a signal transmitted by one of them tothe other, but it could include the case in which a third equipment (forexample NCS managing the two base stations or dedicated third equipment)transmits a synchronisation signal to both base stations.

Note that the invention is not limited to the UMTS or 3GPP networks, andis applicable to any cellular network.

Note that the invention is not limited to a purely hardwareinstallation, but it can also be implemented in the form of a sequenceof instructions in a computer program or in any hybrid form comprising ahardware part and a software part. If the invention is partially orcompletely implemented in a software form, the corresponding sequence ofinstructions could be stored in a storage means that is removable (forexample such as a floppy disk, a CD-ROM or a DVD-ROM) or is notremovable, this storage means being partially or completely readable bya computer or a microprocessor.

1. Method for synchronization of a terminal in a cellular communicationnetwork comprising a plurality of cells, including a first cell and asecond cell associated respectively with a first and a second basestation, said first and second cells at least partly overlapping in acommon geographical area and being synchronized, characterized in thatit includes, when said terminal is present in said common geographicalarea: a first synchronization step on a synchronization signaltransmitted by said first base station and received by said terminal;and a second synchronization step on a predetermined signal dedicated tothe processing of multiple paths transmitted by said second base stationand received by said terminal.
 2. Synchronization method according toclaim 1, characterized in that said terminal comprises a step ofdetermination of at least one path corresponding to said predeterminedsignal transmitted by said second base station, said determination stepsupplying said second synchronization step and being implemented byanalyzing at least a first path along which said predetermined signalpasses, the first path being a synchronization base.
 3. Synchronizationmethod according to claim 1, characterized in that said firstsynchronization step tolerates synchronization errors of the order of 5to 50 μs relative to said second base station.
 4. Synchronization methodaccording to claim 2, characterized in that said first synchronizationstep tolerates synchronization errors of the order of 5 to 30 μsrelative to said second base station.
 5. Synchronization methodaccording to claim 1, characterized in that said second synchronizationstep tolerates synchronization errors of less than 5 μs relative to thesaid second base station.
 6. Synchronization method according to claim1, characterized in that said first base station transmitssynchronization information to said second base station enablingsynchronization of said second base station on said first base station.7. Synchronization method according to claim 6, characterized in thatsaid synchronization information is transmitted on a wire link. 8.Synchronization method according to claim 6, characterized in that saidsynchronization information is transmitted through said synchronizationsignal transmitted by said first base station.
 9. Synchronization methodaccording to claim 1, characterized in that a third piece of equipmenttransmits synchronization information to said first base station and tosaid second base station enabling said second base station to besynchronized on said first base station.
 10. Method of hand-overtransfer of a terminal in a cellular communication network comprising aplurality of cells, including a first cell and a second cell associatedrespectively with a first base station and a second base station, saidfirst and second cells at least partly overlapping in a commongeographical area and being synchronized, and said terminal possiblybeing in communication mode when a communication has been set up betweenthe terminal and a remote terminal, and in standby mode when saidterminal is not in communication mode but is present and is availablefor a communication in one of the network cells, characterized in thatsaid terminal is present in said common geographical area, said methodcomprises: a first synchronization step on a synchronization signaltransmitted by said first base station and received by said terminal; asecond synchronization step on a predetermined signal dedicated to theprocessing of multiple paths, transmitted by said second base stationand received by said terminal; when said terminal in communication modewith said first base station is transferred from said first cell to saidsecond cell.
 11. Hand-over method according to claim 10, characterizedin that it uses a synchronization method that includes the followingsteps, when said terminal is present in said common geographical area: afirst synchronization step on a synchronization signal transmitted bysaid first base station and received by said terminal; and a secondsynchronization step on a predetermined signal dedicated to theprocessing of multiple paths transmitted by said second base station andreceived by said terminal.
 12. Hand-over transfer method according toclaim 10, characterized in that said first cell surrounds said secondcell and in that said first base station manages said standby mode forterminals present in said first cell, said second base station beingable to manage said communication mode.
 13. Hand-over transfer methodaccording to claim 12, characterized in that said first base stationmanages opening of a communication for said terminals present in saidsecond cell, and said network then transfers management of saidcommunication to said second base station.
 14. Terminal for cooperatingwith at least a first base station associated with a first cell in thecellular communication network, said terminal possibly being incommunication mode when a communication is set up between said terminaland a remote terminal, and in standby mode when said terminal is not incommunication mode but is present and available for a communication inone of said network cells, characterized in that it comprises: means ofmaking a first synchronization on a synchronization signal transmittedby said first base station and received by said terminal; means ofmaking a second synchronization on a predetermined signal dedicated toprocessing of multiple paths, transmitted by a second base station andreceived by said terminal; a second cell in said network beingassociated with said second base station; and said first and secondcells at least partly overlapping and being synchronized.
 15. Terminalaccording to claim 14, characterized in that said synchronization meanscomprise means of analyzing multiple paths undergone by saidpredetermined signal, capable of determining at least a first pathcorresponding to said predetermined signal transmitted by said secondbase station, the first path being a synchronization base.
 16. Terminalaccording to claim 14, characterized in that said first synchronizationtolerates synchronization errors of the order of 5 to 50 μs. 17.Terminal according to claim 15, characterized in that said firstsynchronization tolerates synchronization errors of the order of 5 to 30μs.
 18. Terminal according to claim 14, characterized in that saidsecond synchronization tolerates synchronization errors of less than 5μs.
 19. Second base station for association with a second cell incellular telecommunication network, said network comprising a pluralityof cells including a first cell associated with a first base station andat least one terminal, said first and second cells overlapping at leastpartly on a common geographical area, characterized in that said secondbase station comprises means of synchronization on said first basestation such that at least one of said terminals can implement: a firstsynchronization step on a synchronization signal transmitted by the saidfirst base station and received by said terminal; and a secondsynchronization step on a predetermined signal dedicated to theprocessing of multiple paths, transmitted by said second base stationand received by said terminal.
 20. Base station according to claim 19,characterized in that it comprises means of management of acommunication mode of said terminal and that it does not include meansof management of a standby mode of said terminal, said terminal possiblybeing in communication mode when a communication is set up between saidterminal and a remote terminal, and in standby mode when said terminalis not in communication mode but is present and available for acommunication in one of said network cells.
 21. Cellulartelecommunication network comprising a plurality of cells, including afirst cell associated with a first base station and at least one secondcell, said first cell and each of said second cells overlapping at leastpartly in a common geographical area and being synchronized,characterized in that each of said second cells is associated with atleast one second base station according to claim 19.